SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Finance Receivables (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Finance receivable, net	$ 13.1	$ 12.5
Spire Alabama
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold period past due for collection	90 days
Threshold period past due for write-off of financing receivable	12 months
Financing Receivables, Equal to Greater than 90 Days Past Due | Spire Alabama
Accounts, Notes, Loans and Financing Receivable [Line Items]
Finance receivable 90 days past due	$ 0.4	$ 0.4